I don't believe any USB keyboard can achieve true n-key rollover. If you take a PS/2 keyboard that is, and give it a USB converter to make it work on your Mac, then it isn't! It's a USB limitation.
– Adam EberbachApr 5 '12 at 0:03

@AdamEberbach it's not a USB limitation, it is a software driver limitation. Notice that a USB keyboard with a PS/2 adapter can be N-Key rollover. AFAIK the 6-Key rollover limitation only applies to USB on Microsoft Windows (i wonder if anyone has tested it on Windows 8 yet?).
– Abhi BeckertNov 2 '12 at 23:11

5 Answers
5

Current Apple keyboards (as of 2012) are not N-Key rollover and as far as I know no recent keyboard they have ever made is. I think some of the really old ones were N-key rollover, but can't confirm it.

You can use this website to test your keyboard - click on the embedded flash app and press several keys simultaneously

Beware that every keyboard has a different electronic layout and will fail in different key combinations. On my Apple Aluminium keyboard if I hold down the W and E keys, then press the D key, the third key press does not register. But there are other key combinations where as many as six keys will register at once.

It is commonly said that true N-Key rollover can only be achieved using a PS/2 keyboard, and that USB keyboards can only achieve 6-key rollover. As far as I know, this limitation only applies to Microsoft Windows.

My mechanical keyboard's spec sheet claims "6-key rollover with USB, N-key rollover with USB to PS/2 adapter". When using it with USB on my Mac, I've confirmed that is in fact 20-key rollover.

My Apple Wireless Keyboard and the keyboard on my MacBook Air seem to allow pressing all combinations of two keys at the same time, but not some combinations of three keys. So I guess they are 2-key rollover.

Another way to test it is to open the keyboard viewer from the input menu or with open -a KeyboardViewer.

That's not correct, 6-key rollover means any 6 keys can be detected independently, which is not the case for W, D and E on my keyboard (MacBook Pro, late 2011). But it is apparently 2-key rollover.
– Jan SegreSep 13 '15 at 20:27

I was just surprised to see that the keyboard on the Macbook Pro can pass my quick keyboard test. I typed out "The quick brown fox jumps over the lazy dog" while holding down both SHIFT keys. Perhaps the internal keyboard is hooked up via PS/2 or something similar.

However, Apple's external USB keyboards cannot do type that out without missing keys.

As someone who has built a Hackintosh (a computer that is made of standard, off the shelf PC components, but runs Mac OS), I can confirm that Apple's laptops use a PS2 interface. Among the many .kext files that the custom boot loader can install are PS2 drivers. These allow the use of PS2 keyboards and mice with Mac OS, but are derived from .kexts that are used by Apple's laptops.